Chapter hvac engineering Fundamentals: Part 1 Introduction

6.2.2
Pressure reduction
When steam is distributed from a central plant, it may be desirable
to use higher pressures for distribution, resulting in smaller piping.
Then it is usually necessary to use
pressure-reducing valves
(PRVs) to
provide a suitable point-of-use pressure. A typical pressure-reducing
station is shown in Fig. 6.1. To provide better control, it is common
practice to use two PRVs in parallel, one sized for one-third and the
other sized for two-thirds of the load, respectively, and sequenced so
that the smaller valve opens first. This allows the larger valve to work
against smaller pressure differentials, which helps to avoid wire draw-
ing of the valve seat at low loads. A manual bypass with a globe valve
is provided for emergency use. The PRV should have an internal or
external pilot, for accurate control of downstream pressure regardless
of upstream changes.
The maximum pressure drop through any steam PRV at the design
flow rate is about one-half of the entering pressure; more exactly, the
ratio of downstream to upstream pressure cannot be less than 0.53.
This is due to the physical laws governing flow of compressible fluids
Design Procedures: Part 4
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